1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an ink jet printing method in which print heads for ejecting multiple color inks are scanned in two opposite directions for color printing. More specifically, the invention relates to an ink jet printing apparatus and an ink jet printing method which can alleviate color variations that occur during a color printing operation performed by reciprocal scanning or bidirectional scanning.
2. Description of the Related Art
In a printing apparatus, particularly an ink jet printing apparatus, an improvement on a color printing speed has become an important issue. Among possible methods for improving the printing speed are an increasing of the length of print heads and an enhancement of print head drive frequency. In a serial type printing apparatus, which performs printing by scanning the print heads over a print medium, the method of improving the printing speed also includes a bidirectional printing that performs printing not just during a scan in one direction, for example a forward scan, but also during a backward scan. The bidirectional printing is characterized in that an energy required to produce the same throughput is more distributed over time than a unidirectional printing, and thus is advantageous in terms of cost as a total system.
The bidirectional printing, however, has a fundamental problem that since an order in which color inks land on the print medium differs between the forward scan and the backward scan depending on the construction of the print heads, the overlapping order of color inks also differs, resulting in band-like color variations. This problem stems from the order in which color inks are ejected, so if different color dots overlap each other at all, this problem will emerge more or less in the form of a color difference. When a colorant, such as pigment or dye ink, is ejected onto a print medium to form an image, ink dots, after they have landed on the print medium, soak into portions of the print medium ranging from a surface layer to an interior of the medium. Next, when an ink to form subsequent dots is ejected to overlie at least partially the preceding dots on the print medium, a large part of the subsequently ejected ink penetrates and fixes below those portions already colored by the preceding ink dots. As a result, the color of the preceding ink dots tends to show more strongly than the color of the subsequently applied ink. For this reason, in a printing apparatus in which ejection nozzles of different colors are arranged in the main scan direction, performing the bidirectional printing results in band-like color variations because a color ink ejection order during the backward scan is reverse to that during the forward scan. This phenomenon similarly occurs not only with inks but also with wax-based colorants used to produce process colors because of the inverted order of color ink ejection, although there are different working principles behind the phenomenon for different types of colorants.
To solve this problem, the following methods have been proposed. In a first method, two sets of print heads for applying cyan (C), magenta (M) and yellow (Y) inks are arranged symmetrically with respect to the scan direction so that a plurality of secondary color pixels formed along a raster direction have different orders of ink application. Because a plurality of secondary color pixels arranged in the raster direction have different orders of ink application, color variations can be reduced by uniformly distributing image data between the paired, symmetrically arranged print heads to make dots with different ink application orders occur at a constant probability, whether the pixels are formed during the forward scan or the backward scan. Since the image data is allocated uniformly to the paired print heads, there are no impartial concentrations of the number of heating (ejection) operations, i.e., the load on heaters in the print heads can be spread between the print heads.
As an embodiment implementing this method, a technique has been proposed which shifts the paired print heads one-half pitch from each other in the sub-scan direction. With this technique, particularly in a low-pass printing in which color variations easily show up, it is possible to reduce the number of printing (driving) frequency of the print heads and, when a predetermined number of dots are to be arranged in one pixel, arrange these dots in a diagonal positional relation that offers an efficient dot coverage rate.
A second method proposes to perform a multipass printing using the paired, symmetric print heads described above. With this method, complementary masks used in dividing the print data are uniformly allocated to the forward and backward scans to reduce color variations even in the multipass printing.
In the conventional methods that use two sets of print heads symmetrically arranged in the main scan direction and which shifts the paired print heads one-half pitch from each other in the sub-scan direction and distributes the print data uniformly to the paired print heads, the spread processing is performed to distribute the print data to nozzle columns of interest to equalize a probability of dot formation among secondary color pixels arranged in the raster direction. This spread processing, however, may cause unwanted fine textures due to dot arrangement interferences.
These textures easily show mainly on print media with a low bleeding rate, particularly on high quality image printing media with an ink receiving layer on the surface. In a low- to mid-tone range, the textures give a granular impression, degrading an image quality significantly.
The present invention has been accomplished to overcome the aforementioned problems experienced with the conventional methods. In an ink jet printing apparatus using a so-called multipass printing mode in which an image is formed by main-scanning different nozzle groups or nozzle columns over the same scan area a plurality of times, it is an object of this invention to provide an ink jet printing apparatus and an ink jet printing method which can reduce color variations resulting from alternating scan directions and a granular impression called texture produced in a low- to mid-tone range due to the dot arrangement interferences even if a bidirectional printing is performed.
According to one aspect, the present invention provides an ink jet printing apparatus for printing a color image by using a print head having a plurality of arrayed print elements, by scanning the print head over one and the same scan area a plurality of times and by applying a plurality of color inks from the print elements to a print medium in both forward and backward directions of the scan; the ink jet printing apparatus comprising: the print head having, for each ink color, two print element array portions, each having a plurality of print elements arrayed at a predetermined interval, the print elements in one of the two print element array portions being arranged symmetric to the print elements of the other print element array portion with respect to the scan direction of the print head, the print elements in one of the two print element array portions being shifted one-half the predetermined interval from the print elements of the other print element array portion in a direction of array of the print elements; a spread pattern to arrange secondary color pixel data uniformly in a raster direction; a data spreading means to generate the pixel data according to the spread pattern; and a spread pattern changing means to change the spread pattern used by the spreading means according to a state of print data.
According to another aspect, the present invention provides an ink jet printing apparatus for printing a color image by using a print head having a plurality of arrayed print elements, by scanning the print head over one and the same scan area a plurality of times and by applying a plurality of color inks from the print elements to a print medium in both forward and backward directions of the scan; the ink jet printing apparatus comprising: the print head having, for each ink color, two print element array portions, each having a plurality of print elements arrayed at a predetermined interval, the print elements in one of the two print element array portions being arranged symmetric to the print elements of the other print element array portion with respect to the scan direction of the print head, the print elements in one of the two print element array portions being shifted one-half the predetermined interval from the print elements of the other print element array portion in a direction of array of the print elements; a spread pattern to arrange secondary color pixel data uniformly in a raster direction; a data spreading means to generate the pixel data according to the spread pattern; a spread pattern changing means to change the spread pattern used by the spreading means according to a state of print data; and a mask means to mask some of the pixel data generated by the spreading means which correspond to a column located at a predetermined position in the raster direction.
According to still another aspect, the present invention provides an ink jet printing method using an ink jet printing apparatus, wherein the ink jet printing apparatus prints a color image by using a print head having a plurality of arrayed print elements, by scanning the print head over one and the same scan area a plurality of times and by applying a plurality of color inks from the print elements to a print medium in both forward and backward directions of the scan, wherein the print head has, for each ink color, two print element array portions, each having a plurality of print elements arrayed at a predetermined interval, the print elements in one of the two print element array portions being arranged symmetric to the print elements of the other print element array portion with respect to the scan direction of the print head, the print elements in one of the two print element array portions being shifted one-half the predetermined interval from the print elements of the other print element array portion in a direction of array of the print elements; the ink jet printing method comprising: a data spreading step to generate pixel data by using a spread pattern, the spread pattern being used to arrange secondary color pixel data uniformly in a raster direction; and a spread pattern changing step to change the spread pattern used in the spreading step according to a state of print data.
With the above construction, in pixel areas of process colors including secondary colors, the spread pattern is changed according to the state of the print data, so that an image formed has dots of secondary colors evenly distributed. This in turn helps prevent color variations and reduce textures that would otherwise occur in a half-tone range.
In this specification the word xe2x80x9cprint mediumxe2x80x9d refers not only to paper used in general printing apparatus but also to a wide range of media that can receive ink, such as cloth, plastic films and metal plates. The word xe2x80x9cInkxe2x80x9d refers to any liquid that is applied to the print media to form images, designs and patterns or to process the print media. Further, the xe2x80x9cpixel areaxe2x80x9d refers to a minimum area which is applied with one or more inks to produce primary or secondary colors. The pixel area includes not only pixels but also superpixels and subpixels. The number of times that the pixel areas need to be scanned for completion is not limited to once but may be two or more times. Further, the xe2x80x9cprocess colorxe2x80x9d includes secondary colors and means those colors which are produced by mixing three or more color inks on a print medium.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.